This application claims the priority of German application 197 26 216.3, filed in Germany on Jun. 20, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a spindle for spinning or twisting machines comprising a step bearing sleeve as well as a damping tube which takes up said step bearing sleeve, which damping tube is arranged radially movable in a bearing housing with clearance to an oil filled damping gap.
In the case of a spindle of this type (Swiss patent 552 689), the damping tube is a bearing tube fixed in a bearing housing, which bearing tube comprises a flectional area and a damping area. Due to its resilience in the flectional area, the bearing tube makes oscillations in the damping area, which are hydraulically dampened. The readjusting forces for centering the step bearing sleeve are primarily generated by the lateral forces. In order that the radial movements necessary for an effective damping action are possible, the bearing tube must have sufficient clearance in the damping area.
In the case of another spindle of the above mentioned type (British patent 977 278), the damping tube is arranged so that it oscillates with the aid of a tension spring in the bearing housing. Thus in this case a purely mechanical centering of the step bearing sleeve also takes place. In order to achieve a hydraulic damping, radial oscillations are again necessary, which require a sufficiently wide damping gap.
Large damping gaps, present in both cases of the known spindles, lead inevitably to relatively large oscillations of the spindle shaft and thus to an unsteady operation of the spindle. In the case of both known spindles, the damping tube is fixed at a point in the bearing housing, so that the centering forces are generated by means of this fixing point.
It is an object of the present invention to reduce overall the radial movements of the damping tube and thus those of the spindle so that the spindle operates more steadily, and to permit, in addition to the hydraulic damping, a purely hydraulic centering.
This object has been achieved in accordance with the present invention in that the damping tube is arranged free floating in the bearing housing.
Due to the damping tube being floatingly arranged, it has no direct contact with the bearing housing. A torsion lock may be present which prevents a gradual rotation of the damping tube in the bearing housing in certain preferred embodiments. Under no circumstances should any lateral forces be transmitted into the damping tube by means of any torsion lock which may be present. Thus a purely hydraulic centering also takes place in spite of very slight radial movements in addition to a purely hydraulic damping. A so-called gap damping and gap centering occurs for the spindle, whereby a hydrodynamic over-pressure occurs in the oil between the damping tube and the bearing housing as a result of the rotating load in the damping gap. The damping tube is raised from the metal surface of the bearing housing and is supported by the oil film.
The oil pressure in the damping gap depends on the gap width as well as length and the average diameter, and, to a certain degree, on the speed with which the circumferential load moves along the damping gap in circumferential direction. For this reason it is advantageous when the damping tube has the largest possible diameter. Not only is the length of the damping gap enlarged in circumferential direction but also the circumferential speed of the occurring circumferential load is accordingly increased. The gap width can then be made wider.
When in operation, a balance exists between the hydrodynamic oil pressure in the damping gap and the dynamic and static forces, generated by the spindle upper part, acting on the bearing. The damping gap is most effective the further away it is from the neck bearing. For this reason, the damping tube is extended so that it is still arranged underneath the step bearing sleeve.
In order to permit an effective hydraulic damping as well as a hydraulic centering, the damping tube should be as light as possible. It is thus provided that the damping tube is preferably made of plastic or a light metal.
Due to the constant imbalances of the upper spindle part, the step bearing sleeve and thus also the damping tube carry out wobbling type movements, which correspond to the spindle speed. In the case of high spindle rotations, it can happen that gas bubbles form in the circumferential gap narrowing due to the oscillations of the damping tube, and because of the rotating vacuum occurring in the damping gap, the oil starts to evaporate. It is important therefore to ensure that at such points, oil is sufficiently quickly supplied so that the hydraulic centering does not fail. For this reason, in a further embodiment of the invention, the damping tube is provided with radial bore holes which connect the damping gap with the area of the step bearing sleeve. This arrangement, however, requires that the step bearing is lubricated with the same oil as that which is in the damping gap.
For the purposes of the invention, two or four radial bore holes are distributed uniformly in circumferential direction of the damping tube. Additionally the damping gap may extend to form oil pockets in the area of the radial bore holes, in order to permit a particularly quick supply of oil to the critical points. In the case of longer damping tubes it is particularly advisable to provide a plurality of rows of radial bore holes in axial direction of the damping tube.
In order to guarantee a purely hydraulic centering in addition to the hydraulic damping of the step bearing sleeve, the damping gap should not be too large. As a result of tests, it has been established as favorable when the damping gap--with an average diameter of approximately 15 to 20 mm and a length of approximately 35 to 50 mm--has a gap width of from 0.2 mm to 0.4 mm.
In the case of variations from these dimensional recommendations, in particular in the case of a deviation from the gap width, the following disadvantages may occur:
In the case of the damping gap being too small, there is the risk that the shaft of the spindle deflects, that the damping is not sufficient and that the spindle hums during operation. In the case of the damping gap being too large, the shaft of the spindle makes excessively large oscillations, whereby the resetting force is reduced and the spindle operates unsteadily overall. A basic principle to be observed is that the smaller the average diameter is or the shorter the length of the damping gap is, the smaller the gap width must be.